FIELD-LENS. 
image o' o', at a less distance from the object-glass l l. The 
dimensions of this image are so small, that every part of it can be 
seen at once with the eye-glass. 
The portion of the image which can be seen at once with the 
eye-glass, is called the field of view of the microscope. 
It is evident from what has been stated, that the effect of the 
lens f F is to increase the field of view, since by its means the 
entire image of the object can be seen, while without its inter¬ 
position the central parts only would be visible. 
The lens f f has, from this circumstance, been called the 
FIELD-LENS. 
But the increase of the field is not the only effect of this 
arrangement. 
The light which would have been diffused over the surface of 
the larger image o o, is now collected upon that of the smaller 
image o' o'; and the brightness, therefore, will be increased in the 
same proportion as the surface of o o is greater than the surface of 
o' o', that is, in the proportion of the square of o o to the square 
of o' o'. 
Another effect of the field-lens is to diminish the length of the 
microscope, for the eye-glass, instead of being placed at its focal 
distance from o o, is now placed at the same distance from o' o'. 
7. In this brief exposition of the general principle of the micro¬ 
scope, the image, which is the immediate subject of observation, 
is supposed to be produced by a convex lens; such an image, 
however, may also be produced by a concave reflector, and being 
so produced may be viewed with an eye-glass, exactly in the same 
manner as when produced by a convex lens. 
Microscopes have accordingly been constructed upon this 
principle, and are distinguished as deflecting micdoscopes ; 
those in which the image is produced by a lens being called 
DEFDACTING MICDOSCOPES. 
The principle of a reflecting microscope will be understood by 
reference to fig. 3, where l l is the concave reflector, of which c 
is the centre; the object o is placed towards the reflector, at a 
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